Method of producing a steel with high strength, high ductility and good weldability

ABSTRACT

A process for treating steel containing up to 0.25% C, up to 0.5% Si, about 0.5 to about 2% Mn, 0 to 0.1% Nb, 0 to 0.2% V, 0 to 0.2% Ti and 0 to 0.1% Al, balance iron and impurities comprising heating the steel to the austenitizing point, thereafter immediately cooling said steel to the transformation temperature in a lead bath having a temperature of about 350*C. and cold working said steel to achieve an area reduction of 0.5 to 10% whereby a steel having the properties of high strength, good formability and good weldability is produced.

Uited States Patent Pernstal Sept. 2, 1975 [54] METHOD OF PRODUCING A STEEL WITH 3,615,925 lO/l97l Garber et a1 148/12 HIGH STRENGTH, HIGH DUCTILITY AND 3,620,717 11/1971 Sekino et a1. 75/123 .1 3,725,049 4/1973 Satoh et a1. 75/123 J GOOD WELDABILITY 3,726,723 4/1973 Coldren et a1. 148/12 Inventor: Jan Pernstal, Kopparberg, Sweden SKF Industrial Trading and Development Company B.V., .lutphaas, Netherlands Filed: July 6, 1973 Appl. N0.: 377,125

Assignee:

Foreign Application Priority Data July 10, 1972 Sweden 9072/72 US. Cl. 148/12 F; 148/l2.l; 148/l2.4

Int. Cl. C21D 7/02 Field of Search 148/12, 12.4, 12.1;

References Cited UNITED STATES PATENTS l/l97l Boni 75/123 .1

Primary Examiner-W. Stallard Attorney, Agent, or Firm-Howson and Howson [5 7] ABSTRACT A process for treating steel containing up to 0.25% C, up to 0.5% Si, about 0.5 to about 2% Mn, 0 to 0.1% Nb, 0 to 0.2% V, O to 0.2% Ti and 0 to 0.1% A1, balance iron and impurities comprising heating the steel to the austenitizing point, thereafter immediately cooling said steel to the transformation temperature in a 4 Claims, No Drawings 1 METHOD OF PRODUCING A STEEL WITH HIGH STRENGTH, HIGH DUCTILITY AND GOOD WELDABILITY DESCRIPTION OF THE INVENTION This invention relates to a method of making steel having the qualities of high strength, good ductility and good weldability. The steel industry is continually seeking a solution to the problem of making a steel which simultaneously has high strength, good ductility and good weldability. Procedures known heretofore, however, have produced steels having only two of the above mentioned qualities simultaneously, which, of course, has limited the fitness of the steel. Through the practice of the present invention it is possible to produce steel which has all the qualities mentioned.

According to the present invention, steel having the following composition:

up to 0.25% C, up to 0.50% Si, about 0.5 to about 2.0% Mn, 0.l% Nb, 00.27c V, 00.2% Ti, 00.l% Al, balance iron, a steel, the qualities of which are already known, in the form of hot or cold-rolled strips is heated to the austenitizing point at a temperature of 950 960C. and immediately thereafter cooled to the transformation point at a temperature of 300C. to 650C. to form a structure mainly consisting of bainite and ferrite with elements of martensite and fine lamellar perlite. Cooling and transformation is suitably carried out in a lead bath maintained at a temperature of 350C. through which the austenitized strip is fed and where cooling to a temperature of about 300C. occurs at a controlled rate of about 0.6C./sec. to 60C./sec. Subsequently the strip is cold-worked with an areareduction of 0.5- by volume. In the cases where strips are used as basic material, the cold-working operation is carried out suitably as a cold-rolling. Despite the small reduction in surface area occasioned by the cold-working, the yield-point of the steel is considerably increased in the material in question with this procedure without any mentionable decrease of the ductility.

The invention will now be described according to the following example.

As a basic material a cold-rolled strip was used with the following dimensions: 340 X 1.53 mm. The composition of the basic material was:

0.17% C, 0.36% Si, 1.50% Mn, 00.27% Nb, 0%V, 0% Ti, 00.3% Al, balance iron.

This material was subject to a special heat treatment in a continuous vertical furnace. The heat treatment resulted in a mixed structure consisting mainly of bainite and ferrite with elements of martensite and finelamellar perlite.

Thereafter, the material was subject to a cold-rolling with a reduction of thickness from 1.53 to 1.50 mm.

The treatment resulted in the following tensile properties:

01, 79 Kp./mm-

After test welding, the steel had a hardness in the areas influenced by the heat generated by the welding procedure in the range -90%- of the initial hardness which must be regarded as a good weldability.

Compared with steel qualities known heretofore, a steel produced according to this invention has excellent formability, that is, a good elongation in proportion to the high strength. Moreover, it has a good weldability which means a broader range of applicability for the steel. The steel produced according to the invention combines in a way not known heretofore the three properties aimed at namely high strength, good formability and good weldability.

Modification of the invention otherthan using strips as a basic material and cold-rolling as cold-working can be made of course without departing from the invention. For instance, steel wire can be used as basic material which at cold-working is cold-drawn or cold-roller.

The scope of the invention therefore is limited by the appended claims wherein what is claimed is:

l. A method for treating steel containing a maximum of 0.25% C, a maximum of 0.5% Si, about 0.5 to about 2% Mn, 0 to 0.1% Nb, 0 to 0.2% V, 0 to 0.22 Ti, 0 to 0.1% Al, balance iron and residual impurities, comprising 1. heating said steel to a temperature sufficient for said steel to be substantially completely austenitized,

2. immediately cooling said steel to a transformation temperature in a range of about 300C. to 650C. at a cooling rate ranging from 0.6C./see. to 60C./sec. whereby a steel is produced having a structure of mainly bainite and ferrite with elements of martensite and fine-lamellar pearlite and,

3. cold working said steel to achieve an area reduction of 0.5 to 10%, said steps producing a steel having the properties of high strength, good formability and good weldability.

2. The process according to claim 1 wherein said steel is heated to a temperature of about 950960C. to achieve austenitization and cooled to said transformation temperature in a lead bath maintained at a temperature of about 350C.

3. The process according to claim 1 wherein said treated steel has a yield strength of about 79 Kp/mn 4. A steel having the properties of high strength, good formability and good weldability produced by treating a steel containing a maximum of 0.25%C, a maximum of 0.5% Si, about 0.5 to 2% Mn, 0 to 0.1% Nb, 0 to 0.2% V, 0 to 0.2% Ti, 0 to 0.1% Al, balance iron and residual impurities by Y 1. heating said steel to a temperature sufficient for said steel to be substantially completely austenitized,

2. immediately cooling said steel to a transformation temperature in a range of about 300C. to 650C. at a cooling rate ranging from 0.6C./sec. to 60C./sec. whereby a steel is produced having a structure of mainly bainite and ferrite with elements of martensite and fine-lamellar pearlite and,

3. cold working said steel to achieve an area reduction of 0.5 to 10% in said steel.

l l l 

1. A METHOD FOR TREATING STEEL CONTAINING A MAXIMUM OF 0.25%C. A MAXIMUM OF 0.5% SI, ABOUT 0.5 TO ABOUT 2% MN. 0 TO 0.1% NB, 0 TO 0.2%V, 3 TO 0.22TI, 0 TO 0.1% AL, BALANCE IRON AND RESIDUAL IMPURITIES, COMPRISING
 1. HEATING SAID STEEL TO A TEMPERATURE SUFFICIENT FOR SAID STEEL TO BE SUBSTANTIALLY COMPLETELY AUSTENITIZED
 2. IMMEDIATELY COOLING SAID STEEL TO A TRANSFORMATION TEMPERATURE IN A RANGE OF ABOUT 300*C. TO 650*C. AT A COOLING RATE RANGING FROM 0.6*C./SEC. TO 60*C./SEC. WHEREBY A STEEL IS PRODUCED HAVING A STRUCTURE OF MAINLY BAINITE AND FERRITE WITH ELEMENTS OF MARTENSITE AND FINE-LAMELLAR PEARLITE AND,
 2. immediately cooling said steel to a transformation temperature in a range of about 300*C. to 650*C. at a cooling rate ranging from 0.6*C./sec. to 60*C./sec. whereby a steel is produced having a structure of mainly bainite and ferrite with elements of martensite and fine-lamellar pearlite and,
 2. The process according to claim 1 wherein said steel is heated to a temperature of about 950*-960*C. to achieve austenitization and cooled to said transformation temperature in a lead bath maintained at a temperature of about 350*C.
 2. immediately cooling said steel to a transformation temperature in a range of about 300*C. to 650*C. at a cooling rate ranging from 0.6*C./sec. to 60*C./sec. whereby a steel is produced having a structure of mainly bainite and ferrite with elements of martensite and fine-lamellar pearlite and,
 3. cold working said steel to achieve an area reduction of 0.5 to 10% in said steel.
 3. cold working said steel to achieve an area reduction of 0.5 to 10%, said steps producing a steel having the properties of high strength, good formability and good weldability.
 3. COLD WORKING SAID STEEL TO ACHIEVE AN AREA REDUCTION OF 0.5 TO 10%, SAID STEPS PRODUCING A STEEL HAVING THE PROPERIES OF HIGH STRENGTH, GOOD FORMABILITY AND GOOD WELDABILITY.
 3. The process according to claim 1 wherein said treated steel has a yield strength of about 79 Kp/mn2.
 4. A steel having the properties of high strength, good formability and good weldability produced by treating a steel containing a maximum of 0.25%C, a maximum of 0.5% Si, about 0.5 to 2% Mn, 0 to 0.1% Nb, 0 to 0.2% V, 0 to 0.2% Ti, 0 to 0.1% Al, balance iron and residual impurities by 